The subject of air traffic control and collision avoidance schemes has drawn much public and private concern. Currently, the U.S. Government through the Federal Aviation Administration (FAA) operates a nationwide system of ground stations which includes a search radar unit and a radar beacon unit which jointly and independently feed a common digitizer with information about aircraft in the vicinity. This system is known as the air traffic control radar beacon system (ATCRBS).
The information gathered by ATCRBS is sent to a host computer via a series of high speed modems. The host computer also known as the central computer complex (CCC) or air traffic control center (ATCC) host computer. The ATCC host computer then computes navigational and air traffic information to the air traffic controllers' display terminal. The air traffic controller then relays this information to aircraft in the immediate vicinity. More detailed information about the ATCRBS is found in Stocker, U.S. Pat. No. 4,197,538, col. 1 11. 23-46 and Schneider, U.S. Pat. No. 4,161,729, col. 1, 11. 14-25 cited parts of which are both incorporated herein by reference.
The search radar unit gathers information on all aircraft in the vicinity by sending out radio waves and keeping track of the reflections from same. Thus the search radar finds out whether an aircraft is in the vicinity and its azimuth and range. The radar beacon unit duplicates the azimuth and range information of the search radar and interrogates the transponder on-board an aircraft obtaining the aircraft transponder identification number as well as other information. This is standard FAA format.
The information from both units is fed in parallel to the common digitizer where it is placed in a form suitable for transmission to the ATCC. Along with position information the common digitizer adds system overhead such as idle words to separate each interrogation.
The FAA is about to invoke rules which will make the TCAS-I and TCAS-II (Traffic Collision and Avoidance System) mandatory on all common carriers major airline carriers having more than 20 passengers. The TCAS-II system is estimated to cost private airlines $200,000 for each unit. Each on-board unit alone will cost approximately $100,000. The public cost of TCAS-II is estimated at over $800 million.
Problems other than cost associated with the proposed TCAS-II are that the interrogation and reply channels of ATCRBS will become overloaded. The TCAS-II system goes through an "all-call" sequence to determine transponder identification of all transponders in the vicinity. After getting the transponder identification number, TCAS-II adds the number to the role call and all identified Mode S transponders are locked out future "all call" sequences. However, Mode A and Mode C transponder can not be locked out and will continue to respond to the "all call" sequence, adding to the system burden.
Additionally, aircraft without transponders will not be detected by the system. There are approximately 5,000 such general aviation aircraft. In the recent past these type of aircraft have been involved in a significant number of near misses or actual collisions.
Finally, TCAS-II will be of limited range. This will make it exceedingly difficult for the aircraft to make horizontal maneuvers. Greater range is needed to give pilots more to make such maneuvers.
Others have proposed less expensive alternatives to TCAS-II. For example, Crow in U.S. Pat. No. 4,454,510 proposes using a two-way data communication link between each of a plurality of controlled aircraft and the ATCC host computer. While this system is advantageous over TCAS-II because it does not add signals to the ATCRBS environment, it still relays on the ATCC host computer, placing an unwanted burden on the critical system. When traffic control is needed most will be when traffic is the heaviest. This precisely when the ATCC host computer will be taxed to its limit. That makes any further load placed on the ATCC critical which could cause even Crow to fail when it will be needed the most.
Baldwin in U.S. Pat. No. 4,293,857 proposes an aircraft avoidance system which allows aircraft to communicate directly with one another. This system will require the expensive Mode S transponders to be aboard each aircraft. In addition, Baldwin requires each aircraft to having relatively large computers on-board to do the required calculations and to do them fast enough to make the system work. This will mean substantially increased expenses for each aircraft as well as additional weight. Both weight and expense are mentioned as problems in Crow.
Schneider in U.S. Pat. No. 4,161,729 proposes the addition of indicator control panel and a separate receiver and decoder between the transponder and the antenna of the aircraft. Again this means the addition of a substantial expense to each aircraft and the ability to receive only information from participating aircraft.
Stocker in U.S. Pat. No. 4,197,538 discloses a de-centralized system where aircraft communicate directly with one another via a separate on-board FM broadcast and receiving station. Raw data from the common digitizer is broadcast to each of the participating aircraft. Each participating aircraft must then encode the data for display on the participating aircraft's screen. Stocker thus requires expensive additional on-board equipment as well as placing the additional requirement of the ATCC host computer to transmit to the participating aircraft.
Meilander in U.S. Pat. No. 3,668,403 discloses a complex method and algorithm for vehicle traffic control.
Although many have attempted to solve the long felt need discussed in the above references, problems of complexity, high cost and complete data communication remain. What is needed is an aircraft traffic control system that will work well with the existing ATCRBS as well as with future systems. This system should supplement and not place any additional burden on the ATCC host computer. In fact, the preferred system will be invisible to the ATCC host computer. The preferred system should supply even general aviation with complete data on other aircraft in the vicinity.